In the field of optics, a combiner is an optical apparatus that combines two images together, from either the same side of the combiner (reflective/reflective, or transmissive/transmissive) or from the two different sides of the combiner (reflective/transmissive). Often times, optical combiners are used in heads up displays (“HUDs”), sometimes referred to as head mounted displays (“HMDs”) or near-to-eye displays, which allow a user to view a display image superimposed over an external view. The HUD enables the user to view the display image without having to look away from his usual viewpoint. The term HUD originated from its use in avionics, which enabled a pilot to view information while looking forward with his head up, as opposed to looking down at an instrument panel. Conventional HUD combiner implementations include tilted dichroic plates, holographic combiners, transparent substrates used at an angle (e.g., beamsplitters), and zero power shells.
Two versions of combiners exist. The first version combines two fields without adding any optical power to either field (typically a tilted dichroic plate or zero power shell). The second version has optical power, in addition to the combining functionality. The optical power is used to form a virtual magnified image of a microdisplay located at a specific distance from the combiner. Field of view is set by the application requirements. The field of view and the desired microdisplay diagonal determine the focal length (inversely related to the optical power) of the optical combiner.
Holographic combiners are typically used in military applications, due to their significant costs, but do provide a high quality HUD. Holographic combiners can be fabricated by exposing a dichromated gelatin, silver halides, or photopolymers to a pair of intersecting laser beams (reference and object beams). The interference pattern between these beams is recorded into the holographic media thereby forming the holographic combiner after curing. Holographic combiners have a number of drawbacks. They are expensive to fabricated, difficult to mass produce, and have limited life spans (e.g., begin to degrade due to temperature, humidity, pressure and other harsh environmental conditions).
Transparent substrates used at an angle (i.e., a beamsplitter) have been used in automobiles to present the driver with HUD information on the windshield. These optical combiners are made of a clear see-through substrate upon which an external image source displays the CGI. However, since the clear see-through substrate is typically a flat substrate without optical power so as not to distort the external FOV, the clear substrate must be angled (e.g., near 45 degrees) and bulky external magnification lenses are used to expand the display image over the display region. The bulky external lenses and angled nature of the clear see-through substrate combiners do not lend themselves well to compact arrangements, such as HMDs.
Zero-power shells are used to undo the optical power of the combiner such that a ray incident on the combiner from the world side at a particular angle goes out at the same angle towards the eyebox. The term shell merely describes that in see-through mode of operation, there is an inner surface with optical power and outer surface designed to satisfy the zero power condition, combining both surfaces to form a shell.
The above mentioned optical combiners each have their own respective drawbacks, which have limited their use to niche markets. In order to broaden the adoption of an HMD outside of a particular niche market, such an HMD should be implemented with a low cost optical combiner that is compact, lightweight, durable, and provides a good quality image without undue optical aberration.